Stereoselective benzylic C(sp3)-H alkenylation enabled by metallaphotoredox catalysis

Selective activation of the benzylic C(sp3)-H bond is pivotal for the construction of complex organic frameworks. Achieving precise selectivity among C-H bonds with comparable energetic and steric profiles remains a profound synthetic challenge. Herein, we unveil a site- and stereoselective benzylic C(sp3)-H alkenylation utilizing metallaphotoredox catalysis. Various linear and cyclic (Z)-all-carbon tri- and tetrasubstituted olefins can be smoothly obtained. This strategy can be applied to complex substrates with multiple benzylic sites, previously deemed unsuitable due to the uncontrollable site-selectivity. In addition, sensitive functional groups such as terminal alkenyl and TMS groups are compatible under the mild conditions. The exceptional site-selectivity and broad substrate compatibility are attributed to the visible-light catalyzed relay electron transfer-proton transfer process. More importantly, we have extended this methodology to achieve enantioselective benzylic C(sp3)-H alkenylation, producing highly enantioenriched products. The applicability and scalability of our protocol are further validated through late-stage functionalization of complex structures and gram-scale operations, underscoring its practicality and robustness.

Visible-Light-Initiated Air-Oxygenation of Alkylarenes to Carbonyls Mediated by Carbon Tetrabromide in Water

Synthesizing benzyl skeleton derivatives via direct oxidation of functionalized benzylic C-H bonds has received extensive research attention. Herein, a method was developed to prepare carbonyl compounds via photoinduced aerobic oxidation of ubiquitous benzylic C-H bonds mediated by bromine radicals and tribromomethane radicals. This method employed commercially available CBr4 as a hydrogen atom transfer reagent precursor, air as an oxidant, water as a reaction solvent, and tetrabutylammonium perchlorate (TBAPC) as an additive under mild conditions. A series of substrates bearing different functional groups was converted to aromatic carbonyls in moderate to good yields. Moreover, a low environmental factor (E-factor value=0.45) showed that the proposed method is ecofriendly and environmentally sustainable.

Enantioselective Intermolecular Radical Amidation and Amination of Benzylic C-H Bonds via Dual Copper and Photocatalysis

A method for direct access to enantioenriched benzylic amides and carbamate-protected primary benzylamines by C-H functionalization is reported. The C-H substrate is used as limiting reagent with only a small excess of the unactivated amide or carbamate nucleophile. The enantioselective intermolecular dehydrogenative C-N bond formation is enabled by a combination of a chiral copper catalyst, a photocatalyst, and an oxidant, and it takes place under mild conditions, which allow for a broad substrate scope. The method is compatible with late-stage C-H functionalization, and it provides easy access to ¹⁵ N-labeled amides and amines starting from cheap ¹⁵ NH₄ Cl.

Directed, Remote Dirhodium C(sp3)-H Functionalization, Desaturative Annulation, and Desaturation

Iminodirhodium reactive intermediates generated in situ from O-tosyloximes using Rh₂(esp)₂ in CH₂Cl₂ at rt were exploited for an agile trichotomy of challenging transformations: (1) remote C-H functionalizations using an exceptionally broad diversity of inorganic and organic nucleophiles including several unconventional examples, for example, ethers and acyl silanes; (2) desaturative annulation, a biomimetic 1,3-methylene C-C ring-closure with an overall loss of two hydrogens; and (3) directed desaturation for the acceptor-less, regioselective creation of γ,δ- or γ,δ,ε,ζ-olefins. Compared with typical iminyl transition-metal-mediated and 1,5-hydrogen atom-transfer (1,5-HAT) processes, iminodirhodium intermediates are largely underexplored, especially with respect to C(sp³)-H centers and, yet, have the potential to be transformative by virtue of their substrate breadth, regiocontrol, and elusive reaction modality. A substrate scope includes benzylic, allylic, propargylic, tertiary, and α-alkyloxy centers.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

HCl-Catalyzed Aerobic Oxidation of Alkylarenes to Carbonyls

The construction of C-O bonds through C-H bond functionalization remains fundamentally challenging. Here, a practical chlorine radical-mediated aerobic oxidation of alkylarenes to carbonyls was developed. This protocol employed commercially available HCl as a hydrogen atom transfer (HAT) reagent and air as a sustainable oxidant. In addition, this process exhibited excellent functional group tolerance and a broad substrate scope without the requirement for external metal and oxidants. The mechanistic hypothesis was supported by radical trapping, ¹⁸ O labeling, and control experiments.